Mechanism of Hydrocarbon Accumulation and Enrichment of Tuffaceous Tight Oil With Separate Reservoir and Source Rock: A Case Study of Tuff Reservoir From the Permian Tiaohu Formation in the Santanghu Basin, NW China

Abstract

Different from most tight oil reservoirs, the tuffaceous tight oil of the Tiaohu Formation is not in situ oil and neither is there close contact between the source rock and the reservoir in the Malang Sag (Santanghu Basin, China). Based on an integrated analysis of oil–source rock correlation, reservoir characteristics, and rock wettability, combined with a comprehensive analysis of the geological conditions, this study determined the source, type, and mechanism of hydrocarbon accumulation of this tuffaceous tight oil reservoir.

Oil–source rock correlation with biomarkers and δ¹³C values showed that the crude oil originated mainly from underlying source rocks in the Lucaogou Formation. Both of the saturated hydrocarbons are characterized by high β-carotane, a high gammacerane index, high C₂₉ to C₂₇ sterane ratio, and δ¹³C <-30‰. The oil in the tuffaceous tight reservoir is not indigenous but it has migrated over a long distance to accumulate in the upper reservoir. Faults and fractures at the end of the Cretaceous are the oil migration pathway. Vitric tuff and crystal-vitric tuff constitute the main rock types of the tuffaceous tight reservoir. Matrix-related pores in tuffs mainly comprise interparticle pores between the minerals and dissolution intraparticle pores formed by devitrification. The petrophysical properties of the tuff are characterized by high porosity (5–25%) and low permeability (<1.0 mD). Adsorption of polar components of the oil generated from original organic matter in the tuff leads to wettability of partial lipophilicity, which is the main reason for the hydrocarbon charging and accumulation. Reservoir quality controls hydrocarbon enrichment. Synchronistic matching between the good source rocks within the Lucaogou Fm., high-quality vitric tuff reservoir, and connecting faults and fractures contributed to the formation of the tuffaceous tight oil reservoir. Locations that satisfy such conditions should be the focus of future prospecting.

The Tiaohu Fm. tight oil with separate reservoir and source rock is a special type of tight oil. Commercial oil has been found under the guide of the mechanism of hydrocarbon accumulation and enrichment in the Santanghu Basin. This special type of tight oil reservoir is an excellent example for researchers investigating similar situations in other areas of the world.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018